Problem: The scale on a map is 4cm : 9km. If the distance between two cities is 90km, how far apart in cm are the two cities on the map?
The scale means that every 4cm on the map corresponds to an actual distance of 9km. An actual distance of 90km is the same as 10 $\cdot$ 9km. The distance between the two cities on the map is 10 $\cdot$ 4cm, or 40cm.